1. Field of the Invention
The invention relates to a method for detecting a concealed dielectric object.
2. Discussion of the Background
In security technology, for example, at airports, various technologies have been established for the identification of concealed or covered objects. While ionizing x-ray radiation, which allows the identification of a plurality of objects and materials, is used for monitoring items of luggage and bags, only metal detectors, which detect exclusively objects made of metal are generally used for monitoring people and the clothing worn by people.
In recent years, systems with microwaves and millimeter waves have been developed for detecting objects made of non-metallic materials, which are concealed by people in their clothing. A system and a method for detecting objects made of different materials carried on a human body by means of millimeter waves is specified in U.S. Pat. No. 6,965,340 B1.
Since millimeter waves are reflected only slightly or not at all in the air and the clothing of the person under investigation and experience an approximately total reflection on metallic objects and on objects with a high water content, such as human skin and human tissue, and a partial reflection on objects with a permittivity between that of air and that of water, dependent upon the permittivity and the thickness of the object, an object made from a non-metallic material, for example, a knife made of ceramic or an explosive powder kept concealed in the proximity of the human body, can be identified by measuring the modulus and the phase of the reflected millimeter waves by means of a focusing method.
However, the method described in U.S. Pat. No. 6,965,340 B1 provides the following disadvantage for the detection of objects kept concealed on a human body:
By comparison with a purely planar object, in the case of an object with a non-flat, especially with an “tapered” surface, the reflected millimeter waves are modified because of a relatively higher scattering complexity. In this case, an image of the original object corresponding spatially with the original object and accordingly a spatially unambiguous and accurate detection of the concealed object is no longer guaranteed.